Petroleum refining operations in which crude oil is processed frequently produce residual oils that have very little value. The value of residual oils can be increased using a process known as delayed coking. Residual oil, when processed in a delayed coker, is heated in a furnace to a temperature sufficient to cause destructive distillation in which a substantial portion of the residual oil is converted, or “cracked” to usable hydrocarbon products and the remainder yields a residual petroleum by-product which is pumped into a large vessel known as a coke drum.
The production of coke is a batch process. Each delayed coker unit usually contains more than one coke drum. In delayed coking, the feed material is typical residuum from vacuum distillation towers and frequently includes other heavy oils. The feed is heated as it is sent to one of the coke drums. The feed arrives at a coke drum with a temperature ranging from 870 to 910° F. Typical drum overhead pressure ranges from 15 to 35 PSIG. Coker feedstock is deposited as a hot liquid slurry in a coke drum. Under these conditions, cracking proceeds and lighter fractions produced flow out of the top of the coke drum and are sent to a fractionation tower where they are separated into vaporous and liquid products. A solid, residuum called coke is also produced and remains within the drum. When a coke drum is filled, residual oil from the furnace is diverted to another coke drum. When a coke drum is filled to the desired capacity, and after feedstock is diverted to another drum, steam is typically introduced into the drum to strip hydrocarbon vapors off of the solid material. The material remaining in the coke drum cools and is quenched. Solid coke forms as the drum cools and must be removed from the drum so that the drum can be reused. While coke is being cooled in one drum and while the cooled solid coke is being extracted from that drum, a second drum is employed to receive the continuous production of coke feedstock as a part of the delayed coker process. The use of multiple coke drums enables the refinery to operate the furnace and fractionating tower continuously. Drum switching frequency ranges from 10 to 24 hours.
In typical coking operations dramatic heat variances are experienced by elements in the coking operation. For example, a coke drum is filled with incoming byproduct at about 900 degrees Fahrenheit and subsequently cooled after being quenched to nearly ambient temperatures. Not surprisingly, this repetitive thermal cycling may create or cause significant problems including stark heat distributing variances throughout various components of a valve system. The heated residual byproduct utilized in coking operations comes into contact with not only the coke drum, but valve and seat components. This heating and subsequent cooling may result in expansion of various elements within a valve system. As previously mentioned the delayed coking process typically comprises at least two vessels so that while one is being filled the other is being purged of material and prepared to receive another batch of byproduct. Thus, during the off cycle, when a vessel is being purged of its contents it will cool and return to a state of equilibrium. It is this cyclical pattern of dispensing hot residual byproduct into a coke drum and subsequently cooling the byproduct that leads to thermal differential and stress within the coke drum, a valve, the valve parts or a line. It is this cyclical loading and unloading and stressing and un-stressing of a coke drum, valve or line that is referred to as thermal cycling. Thermal cycling typically results in the weakening or fatiguing of a coke drum, a valve and its parts which may lead to a reduction in the useful life of the components. Uneven heat distributions or thermal variants existing between various components of the seat system result in decreased longevity of the constitutive elements of the valve body.
Also, because coke is formed using pressure, the deheading valve must form a seal to allow the pressure to build within the coke drum. This seal is generally formed using tight tolerances between the components of the deheading valve such as between the seats and the blind. These tight tolerances, however, increase the force required to slide the blind between the seats to open and close the valve. Also, due to this pressure, it is common to pressurize the internal compartments of the deheading valve such as by providing steam to the internal compartment. If a deheading valve does not provide a good seal, large amounts of steam will escape which increases the total amount of steam required. In many cases, the cost of supplying steam to pressurize the valve can be significant.